1. Field of the Invention
The present invention relates to active-matrix liquid-crystal display apparatuses, and more particularly, to the structure of opposed electrodes used in an active-matrix liquid-crystal display apparatus which is provided with a main display area and a sub display area having pixel zones different in size from each other.
2. Description of the Related Art
FIG. 6 shows a conventional active-matrix liquid-crystal display apparatus. More specifically, FIG. 6 is a plan view of a thin film-transistor (hereinafter called TFT) array substrate 140.
The TFT array substrate 140 of the conventional active-matrix liquid-crystal display apparatus is formed of a display area 130 where pixel zones 132 constituting pixels are disposed in a matrix manner, scanning-line leads 134 and scanning-line terminals 136 used for connecting scanning lines 101 disposed in the display area 130 to a gate-driver IC provided outside, and signal-line leads 135 and signal-line terminals 137 used for connecting signal lines 119 disposed in the display area 130 to a source-driver IC provided outside.
In addition to a display area 30 (hereinafter called a main display area), another display area 31 (hereinafter called a sub display area) has been proposed for displaying character information, for example, as shown in FIG. 1. This increases the complexity of the conventional LCD apparatus, but adds functionality as well.
In this case, although the size of each pixel zone 32 is decreased because high resolution is required for the main display area 30, it is not necessary for each pixel zone 33 to have the-same size as each pixel zone 32 in the main display area due to the display purpose of pixels in the sub display area. Rather, each pixel zone 33 in the sub display area 31 is designed such that it is larger than each pixel zone 32 in the main display area 30 since it is required that, for example, characters be displayed large for easy recognition.
In a conventional liquid-crystal display apparatus having a sub display area, one opposed electrode 113 common to the whole surface of an opposed substrate 141 is formed, as shown in FIG. 7.
In each of active-matrix liquid-crystal display apparatuses, a liquid-crystal layer is sandwiched by a pair of substrates disposed oppositely and used as a display medium. An AC voltage on which a DC voltage is not superposed is applied to the liquid-crystal layer to prevent image sticking on the liquid-crystal layer. The AC voltage is used as a display voltage, and is applied to pixel electrodes mainly constituting pixel zones from signal lines through TFTs that have been turned on by gate voltages applied from scanning lines. A constant DC voltage is applied to an opposed electrode disposed oppositely to the pixel electrodes through the liquid-crystal layer. With this operation, an electric field is applied to the liquid-crystal layer to change its refractive index, and thus the liquid-crystal layer can be used as a display medium.
A dynamic voltage drop occurs in the potential Vp of the pixel electrodes when the gate voltages are changed in order to turn off the TFTs because the dielectric constant of the liquid crystal changes according to the electric-field strength, a parasitic capacitance is formed between the gate electrode and the drain electrode of each TFT, and a parasitic capacitance is formed between a scanning line and the pixel electrode.
FIG. 5 is an outlined view of driving voltages for the liquid-crystal display apparatus. In FIG. 5, (a) shows a voltage Vg applied to the gate electrode of a TFT, (b) shows a voltage Vs applied to the source electrode of the TFT, and (c) shows the voltage Vp of the drain electrode of the TFT, namely, the pixel electrode. In (c) of FIG. 5, Vsc indicates the center voltage of an AC voltage applied to the source electrode, and Vcom indicates a voltage applied to the opposed electrode. Since the voltages Vcom and Vp are applied to the opposed electrode and the pixel electrode, respectively, an effective potential is given to the liquid-crystal layer and the liquid-crystal layer operates as a display medium. The horizontal axis indicates time in FIG. 5 to show the Vg, Vs, and Vp timing. The TFT is xe2x80x9conxe2x80x9d while the voltage shown in (a) of FIG. 5 is high, and the TFT is xe2x80x9coffxe2x80x9d while the voltage is low.
When the gate voltage Vg is changed in order to turn off the TFT, a dynamic voltage drop xcex94Vp occurs at the potential Vp of the pixel electrode as shown in (c) of FIG. 5. This is because, when the gate voltage Vg is changed in order to turn off the TFT, charges are distributed among the capacitor formed by the liquid-crystal layer between the pair of substrates; a storage capacitor formed by a scanning line, and a gate insulating film and a capacitor electrode disposed thereabove; and the above-described parasitic capacitors to generate the voltage drop xcex94Vp at the potential Vp of the pixel electrode.
The voltage drop xcex94Vp generated at the potential of the pixel electrode 11 is shown by the following expression (1).
xcex94Vp=(Vghxc3x97(Cgdon+Cgp)xe2x88x92Vglxc3x97(Cgdoff+Cgp)xe2x88x92Vs(Cgdonxe2x88x92Cgdoff))/(Cs+Clc+Cgdoff+Cgp)xe2x80x83xe2x80x83(1)
where,
xcex94Vp: Voltage drop at the potential of the pixel electrode
Vgh: High potential of the gate voltage
Cgdon: Parasitic capacitance obtained when the TFT is xe2x80x9conxe2x80x9d
Cgp: Parasitic capacitance obtained between the scanning line and the pixel electrode
Vgl: Low potential of the gate voltage
Cgdoff: Parasitic capacitance obtained when the TFT is xe2x80x9coffxe2x80x9d
Vs: Potential of the signal voltage
Cs: Storage capacitance
Clc: Capacitance of the liquid-crystal layer
The factors which cause the voltage drop xcex94Vp at the potential of the pixel electrode includes the capacitance Clc of the liquid-crystal layer, the parasitic capacitance Cgd of the thin-film transistor, and the storage capacitance Cs, as shown in the expression (1).
The dielectric constant of the liquid crystal, one factor causing the voltage drop xcex94Vp, changes according to the electric-field strength. This change relates to the characteristics of the liquid crystal. In the two parasitic capacitances, that formed between the TFT gate electrode and the TFT drain electrode and that formed between a scanning line and the pixel electrode, which are other factors causing the voltage drop xcex94Vp, the parasitic capacitance formed between the TFT gate electrode and the TFT drain electrode is a capacitance generated by the gate insulating film formed between the electrodes, and originates from the structure of current active-matrix liquid-crystal display apparatuses.
When the voltage drop xcex94Vp occurs at the potential Vp of the pixel electrode as described above, the positive and negative voltage amplitudes of the potential Vp of the pixel electrode differ. When an identical-amplitude voltage is applied irrespective of its polarity, liquid crystal shows an identical transmittance. Therefore, in a normally-white active-matrix liquid-crystal display apparatus which has a high transmittance when a voltage is not applied, for example, the transmittance is lower at a polarity where the voltage amplitude is larger, and the transmittance is higher at a polarity where the voltage amplitude is smaller. Consequently, the repetition of brightness and darkness occurs according to the transmittances, and this pattern is seen as flicker.
When voltage amplitudes are not symmetrical for the positive and negative polarities, a DC voltage superposed on an AC voltage is always applied to any of pixel electrodes, and an image remains on the screen, which is so-called image sticking.
Therefore, flicker and image sticking are conventionally avoided by adequately adjusting the potential of the opposed electrode such that the voltage amplitudes of the AC voltage driving the liquid crystal are equal at the positive and negative sides and by forming storage capacitors in parallel to the capacitor generated by the liquid-crystal layer.
When a sub display area having different pixel zones from those of a main display area is provided in addition to the main display area, since the values of the liquid-crystal capacitance and the parasitic capacitances differ according to the size of pixel zones, the voltage drops xcex94Vp at pixel electrodes differ between the main display area and the sub display area. As a result, when the common opposed electrode is used as shown in the conventional case although the most-appropriate potentials applied to the opposed electrode differ between the main display area and the sub display area, the most-appropriate voltage is not applied to the opposed electrode corresponding to one of the main display area and the sub display area. Flicker occurs in either the main display area or the sub display area. In addition, image sticking occurs in either the main display area or the sub display area.
Accordingly, it is an object of the present invention to provide an active-matrix liquid-crystal display apparatus having a main display area and a sub display area having pixel zones of differing sizes, which has opposed electrodes for each of the main display area and the sub display area to allow the most-appropriate voltages to be applied to the respective opposed electrode and which is provided with a mechanism that prevents flicker and image sticking.
In other words, an object of the present invention is to provide an active-matrix liquid-crystal display apparatus in which an appropriate opposed-electrode voltage Vcom is applied to each opposed electrode when different voltage drops xcex94Vp occur at the main display area and the sub display area, such that the voltage amplitudes are equal at the positive and negative polarities in each display area.
The foregoing object is achieved in one aspect of the present invention through a provision of an active-matrix liquid-crystal display apparatus including: a pair of substrates disposed oppositely and a liquid-crystal layer sandwiched by the pair of substrates. A plurality of scanning lines and a plurality of signal lines are formed in a matrix on one of the pair of substrates. The thin-film transistors have gate electrodes connected to the plurality of scanning lines, and pixel electrodes and storage capacitors connected to the thin-film transistors, all of which are disposed in the vicinity of intersections of the plurality of scanning lines and the plurality of signal lines. A main display area and a sub display area have pixel zones of different sizes enclosed by the plurality of scanning lines and the plurality of signal lines. A plurality of opposed electrodes are formed between another of the pair of substrates and the liquid-crystal layer, of which one opposed electrode is disposed oppositely to the main display area and another opposed electrode is disposed oppositely to the sub area.
The active-matrix liquid-crystal display apparatus may be configured such that the size of each pixel zone in the main display area is smaller than that of each pixel zone in the sub display area.
The active-matrix liquid-crystal display apparatus may further include a voltage supply to apply different voltages to the opposed electrode disposed oppositely to the main display area and to the opposed electrode disposed oppositely to the sub display area.
In this case, the active-matrix liquid-crystal display apparatus may be configured such that the size of each pixel zone in the main display area is smaller than that of each pixel zone in the sub display area, and the voltage supply applies a lower voltage to the opposed electrode disposed oppositely to the main display area than a voltage applied to the opposed electrode disposed oppositely to the sub display area.
When the size of each pixel zone in the main display area differs from that of each pixel zone in the sub display area, separate voltage supplies are provided as the voltage supply to apply different voltages to the opposed electrodes disposed oppositely to the main display area and the sub display area, according to the size of each pixel zone. One voltage supply may be provided in order to apply different voltages to the opposed electrodes.
When the main display area has a high resolution for displaying images, and the sub display area is used for displaying characters (i.e. lower resolution is acceptable), for example, the size of each pixel zone in the main display area is smaller than that of each pixel zone in the sub display area and a lower voltage is applied to the opposed electrode disposed oppositely to the main display area than a voltage applied to the opposed electrode disposed oppositely to the sub display area.
In other words, in the expression (1), since the size of each pixel zone in the sub display area is larger, the capacitance Clc of the liquid-crystal layer is large and therefore the voltage drop xcex94Vp is small. To make voltage amplitudes in the positive and negative polarities equal in (c) of FIG. 5, a voltage applied to the opposed electrode disposed oppositely to the sub display area is higher than that applied to the electrode disposed oppositely to the main display. With this operation, the most-appropriate voltages are applied to the opposed electrodes according to the size of each pixel zone, and thus flicker and image sticking are avoided.
In addition, a method for decreasing image sticking and flicker in a liquid crystal display is provided. The method comprises providing a main display area having pixels of a first pixel size and a sub display area having pixels of a second pixel size. Voltages determined by pixel size are supplied to each display area.
The method may further comprise displaying character information in the sub display area. The supplying of the voltages to each display area may further comprise providing each voltage through an opposed electrode.